Tech Innovation

The Female Code Crackers of Bletchley Park

code breakers

As British women stepped forward to aid the war effort during WWII, a select few became cryptanalysts and played instrumental roles in decoding Axis messages and helping the Allies win the war.

In some of the darkest days of WWII, when the German Luftwaffe laid swaths of London to waste and U-boats sunk ships carrying troops and supplies, the tide changed for Britain when a group of top secret code crackers first broke the German Enigma ciphers.

Many know that mathematician Alan Turing, “Father of the Modern Computer,” helped achieve that breakthrough. Until the recent cinematic release of The Imitation Game, however, few had any idea that a woman, Joan (Clarke) Murray, was working alongside him. She played a significant role in breaking the “unbreakable” code.

Bletchley Park Code Crackers
Bletchley Park Credit: Shaun Armstrong ©

In fact, many women were behind the top secret Government Code and Cipher School (GCCS) project, carried out 50 miles from London at a Victorian estate known as Bletchley Park. Three in particular – Murray, Margaret Rock and Mavis (Lever) Batey – broke through gender biases of the day to become cryptanalysts.

“Bletchley Park could not have functioned without its female employees,” said David Kenyon, research historian at Bletchley Park Trust, now a museum. “By 1945, there were more than 6,700 women working at Bletchley and its outstations, forming more than 75 percent of the workforce.”


Many women operated the machines and devices used in the codebreaking process. According to Kenyon, that included teleprinters, encryption machines, Hollerith punch card equipment, huge cryptanalytical Bombe machines and the precursor to today’s computers, a machine called “Colossus.”

Wren attends to Colossus code breakers
Colossus, Credit: ©Crown. Reproduced by kind permission, Director, GCHQ

“This work was both physically and intellectually demanding, requiring absolute accuracy in the face of long shifts, unsocial hours, and often unpleasant working conditions,” Kenyon said.

Then there were the rock stars, Murray, Rock and Batey, who devised the techniques by which codes and ciphers could be broken.

Breaking Through

At Cambridge, Murray earned “double firsts” in math, which indicates first-class honors in the same subject on subsequent exams, as women weren’t allowed to earn degrees there until 1948. This achievement caught the attention of Gordon Welchman, a mathematician and GCCS recruiter.

Though Murray was initially assigned clerical duties, it became quickly evident her abilities surpassed her job assignment and pay grade. She joined Turing’s Hut 8 team as a cryptanalyst. Since women could not hold that position, Murray was promoted to “linguist,” allowing a slight boost in compensation.

The Enigma, an electro-mechanical machine used by the German military to encrypt and decrypt messages, looked like a typewriter but, in addition to a keyboard, also included a “lampboard,” three internal rotors (each with twenty-six electrical contacts), and a “plugboard” with cables.

Because of these components, Enigma offered more than “159 million million million” combinations for coding. To keep the Allies from decoding messages, settings were changed daily. The Germans also added rotors to some machines during the war, increasing the number of combinations in the mix.

Recognizing the impossibility of decoding messages by hand while racing against the clock, Turing designed (and Welchman later refined) the Bombe, a giant electromechanical machine designed to replicate the action of 10 Enigmas.

Bombe code crackers
Front view of Standard Bombe, Credit: ©Crown. Reproduced by kind permission, Director, GCHQ

While exponentially faster, even the Bombe required time to sort through billions of possible settings to decrypt a message. To reduce the options, cryptanalysts used intellect and ingenuity.

“Cribbing” involved looking for a pattern of text indicating an expected phrase such as “weather report” or “attack at dawn.” Murray was expert at cribbing, as well as banburismus, a technique which involved using math probabilities to determine the most likely positions of the right and middle rotors in the Enigma machine when a specific cipher was created.

Though some found the work tedious, Murray found it “enthralling.”

Due to their expertise, the cryptanalysts in Hut 8 were given the important task of breaking codes related to German U-boats, which had been particularly destructive to the Allies. As a result of their success, the Allies gained the ability to track U-boats and either sink or circumnavigate them, an invaluable advantage in the Battle of the Atlantic.

Murray also broke codes used by the German railways, as well as complex Offizier messages, and was promoted to Deputy Head of Hut 8 in 1944, a testament to the respect she earned from male counterparts, as few women were given positions of leadership.

Dilly’s Girls

Rock and Batey were also “linguists” at Bletchley, though Rock worked in mathematics, while Batey’s area of expertise was German language. Both worked as cryptanalysts under Alfred Dillwyn “Dilly” Knox, Britain’s leading WWI code breaker and chief cryptographer at Bletchley.

In 1941, Dilly’s Girls, as they were known, broke the Abwehr Enigma used by German Military Intelligence. This allowed Britain to control the German spy network in Britain and feed misinformation back to Hitler about where the Allies would attack on D-Day.

The women also broke the Italian cipher that led to Allied victory in the Battle of Matapan, which knocked the Italian Navy out of the war.

“Churchill was over the moon; he said that there was nothing like it, no fight since Trafalgar, and Cunningham was as good as Nelson and so on,” Batey later said. “And then came the message from the admiralty at midnight: ‘Tell Dilly, we have won a great victory in the Mediterranean, and it’s all thanks to him and his girls.’”

Batey said she and the other women desperately wanted to share these stories with their families, but they were bound to secrecy. As Churchill famously put it on a visit to Bletchley in 1941, they were “the geese that laid the golden eggs but never cackled.”

A Place in History

Before the war was over, Bletchley was home not only to Bombes, but also the Colossus, sometimes referred to as the world’s first fixed-program, electronic, digital computer, used to break the Lorenze SZ40 cipher used by German High Command.

In the end, historians estimate that the work at Bletchley shortened the war by two to three years, saving thousands of lives.

For her contribution, Murray was appointed Member of the Most Excellent Order of the British Empire (MBE) in 1947.

Additionally, the women’s stories are finally being shared as historians and writers pore through recently released war documents and other correspondence made available by the women’s families.

“Women like Margaret Rock, Joan Clarke [Murray] and Mavis Lever [Batey] are marvelous examples of women meeting their potential in a time of conflict, opening the door to long careers in codebreaking that they would never have been exposed to before the war,” said Kerry Howard, Bletchley Park researcher.

“These are inspiring role models for young women considering careers in STEM, where there are more opportunities than the Bletchley women could ever dream of.”


Feature image credit: ©Crown. Reproduced by kind permission, Director, GCHQ

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